P814: ANALYSIS OF TP53 ALTERATIONS BY WGS REVEALS BIALLELIC 17P DELETIONS IN MULTIPLE MYELOMA
Authors: Marietta Truger, Stephan Hutter, Wencke Walter, Xiang Zhou, Christine Riedhammer, Manja Meggendorfer, Heiko Müller, Wolfgang Kern, Torsten Haferlach, Hermann Einsele, K Martin Kortum, Claudia Haferlach, Leo Rasche
Journal: HemaSphere (2023)

Topic: 13. Myeloma and other monoclonal gammopathies - Biology & Translational Research Background: Aberrations of TP53 such as deletions (del), mutations (mut), copy-neutral loss of heterozygosity (cnLOH) and even biallelic events represent a key driver lesion in hematological neoplasms. In multiple myeloma (MM), TP53 del (del17p) herald poor prognosis and a double hit (del17p+TP53mut) was associated with ultra-high-risk disease in prospective trials. The routine diagnostic approach to detect genetic aberrations in MM is FISH, which does not capture the varying nature of TP53 alterations. Thus, we analyzed whole genome sequencing (WGS) data of 110 patients (pts), 35 newly diagnosed (ND) and 75 relapsed/refractory (RR) MM pts, to uncover the entire picture of TP53 lesions in MM. Aims: (1) To analyze mono- and biallelic TP53 alterations by WGS; (2) To evaluate their effect on TP53 expression by whole transcriptome sequencing (WTS), and (3) their impact on overall survival (OS). Methods: WGS (100x, 2x151bp) and WTS (50 Mio reads, 2x101bp) were performed with purified CD138+ cells on NovaSeq instruments. Data were analyzed with a tumor w/o normal pipeline for single nucleotide variants (Strelka2), structural variants (Manta), copy number variations (GATK), cnLOH (HadoopCNV) and expression patterns. Results: In our cohort high-risk genetic abnormalities were significantly enriched with 73% of pts showing at least one of the high-risk markers del17p, t(4;14), t(14;16) and 1q gain. A del17p was detected in 27% (30/110), being the sole TP53 abnormality in only 8% (9/110). One or two TP53mut sole were found in 3 and cnLOH in 2 of 110 pts. A double hit of del17p+TP53mut was identified in 18 cases (16%), with 2 pts harboring two mut, in line with evolution of two subclones with different TP53mut that emerged from a clone with del17p. As expected, TP53 alterations were significantly more frequent in RRMM (41%, 31/75) than in NDMM (11%, 4/35) (p=0.002). OS analysis showed an unfavorable prognosis for del17p and even worse for biallelic TP53 alteration, however without statistical significance in this cohort with heavily pretreated RRMM and high-risk pts. Of most interest, in 3 patients with del17p WGS revealed a small homozygous del affecting TP53 as biallelic event, that was not identified by FISH. To our knowledge, homozygous TP53 del have not been reported so far. In patient 01, a homozygous del resulted in biallelic loss of the entire TP53 gene. In pts 02 and 03, homozygously deleted regions of 33kb and 15kb comprised exons 10 and 11 of TP53 (Fig.A). WTS data based evaluation of TP53 gene expression on the transcript level showed that the canonical TP53 transcript was lost in these pts, while transcription of a shorter isoform was detectable (Fig.B). While pts 02 and 03 had RRMM, patient 01 was ND, but the presence of an IGH::MYC rearrangement – typical for progressed MM – and death within one year suggest an advanced disease. As TP53 alterations are recurrent in other hematological malignancies, we screened our WGS data set of 4542 pts with myeloid and lymphoid neoplasms for biallelic TP53 del. In this large cohort, a homozygous del comprising TP53 was found in just one patient with a mature T-cell neoplasm. Summary/Conclusion: (1) Frequencies of TP53 alterations are increased in RRMM compared to NDMM. (2) Mut as sole TP53 abnormality and 17p cnLOH occur rarely in MM. (3) Homozygous TP53 del with effect on TP53 gene expression was revealed as very rare biallelic event in hematological neoplasms, which may be more prevalent in RRMM. Further studies with large cohorts of RRMM pts have to elucidate the frequency of biallelic TP53 loss, biological consequences and impact on prognosis.Keywords: Multiple myeloma, TP53